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Can a Boeing 737-800 make a smooth landing on a 7000-foot runway?
I recently flew on an Avelo (brand new airline) Boeing 737-800  to KBUR, 6886’ x 150’ runway. It was a VERY rough touchdown, the braking system made a lot of racket, and our foreheads hit the seats in front of us.
On exit, the flight crew just smiled and said “short field airport”—as if that excused them. But we were 5 minutes ahead of schedule, and it was not windy or gusty as we debarked—and the temperature was only 85°F (30°C) and clear. Surely those were conditions to finesse a better flare out etc? I later heard that pilots do sometimes make soft landings there—but shouldn’t good landings be routine?
The terminal has been cited as too close to the runway, and in May FAA gave approval to build a new one. Am I being too critical of Avelo?

Related: What vertical speed at touchdown makes a 'perfect' landing?

"I later heard that pilots do sometimes make soft landings there—but shouldn’t good landings be routine?", surprisingly soft and good are not synonymous, see: Is a firm landing an indication of the pilot inexperience?

@Bianfable: Also you own answer is a good one and specifically deals with low cost airlines and their link with short runways.

Not sure what the terminal being cited as too close to the runway has to do with this...

Avelo usually uses the shorter runway 8 on KBUR, 5802' x 150', so 6886' is relatively long. According to wikipedia all landings of commercial aircraft take place on runway 8.  Note that the 6886' runway, when used from the North (RWY 15) has a 1.2% downslope.

@ymb1, all you have achieved in changing 6886 to 7000 in the title is to make readers scratch their heads. Sonja, if I were you I would change it back (but keep the "-foot").

Try Ryan Air and you are less likely to experience "bad landings" in future. The only aitline n which I've fel hard turning foces while decelerating from landing (Dublin).

wow 737-800s fly in to bob hope airport??? I had no idea about that. amazing!

You're probably being hard on them...

it was not windy or gusty as we debarked (disembarked)

The conditions you feel on the ground can be very different 50 feet in the air. Even from one end of the airport to the other can be different. As you were disembarking, I'm guessing you were near the terminal, which means you may have had some wind shielding as well.

the temperature was only 85 and clear

To you that sounds like a perfect smooth day, we pilots know better. Clear skies and a warm day means a bumpy ride.

I later heard that pilots do sometimes make soft landings there

A common misconception with passengers that a "soft" landing is a good landing, and that just isn't the case, especially with short fields. "Greaser" landings mean you are spending a lot of time bleeding off lift when you could be on the ground and effectively using your aircraft weight and brakes. Light aircraft (soft on the feet) means you can't brake as hard without skidding.

but shouldn’t good landings be routine?

Good landings are those you walk away from. OK landings are when the aircraft can fly again, bad landings require repairs, and everything else is a crash.
Just as a little side note, there may have been some mechanical issues with your aircraft like inoperative thrust reversers requiring them to brake much harder than they normally would. The crew could have also encountered a thermal crossing the threshold making them land a little long.
Or Avelo (I'm not familiar with the airline) could be notorious for those types of landings, I believe another one that has that reputation is Ryan Air.
To answer the title question:

Can a Boeing 737-800 make a smooth landing on 6886’ runway?

The 737-800 has the capability to operate out of fields less than 5000' long, depending on the exact capabilities as ordered. The Short Field option is standard on the 737-900ER, and an option on the -600, -700, and -800.

Also field elevation (density altitude) could be a factor. Would help if the OP would tell us where the field is.

KBUR is Hollywood Burbank @jamesqf.

Good landings are those you walk away from. I take exception to that nonsensical phrase. I have personally walked away unscathed from a heavily damaged aircraft in what I can tell you was definitely not a good landing. And several times I have also witnessed others walk away from written-off aircraft; those were not "good landings" either.

@MartinArgerami It is a famous quote by Chuck Yeager: "If you can walk away from a landing, it's a good landing. If you use the airplane the next day, it's an outstanding landing." (source) Of course it should not be taken literally ;)

@MartinArgerami try not to take it too seriously, it's a long running joke between pilots.

@RonBeyer: you are answering a question from someone who is definitely not a pilot. I doubt they understand the "joke".

@Bianfable: yes, it's a quote from a test pilot. Not exactly the kind of quote needed to reassure the average airline passenger.

I'm not a pilot. I've heard that phrase many times, though never know it was a Chuck Yeager quote. I think that I've mentioned a variation of it to my passengers once as an over-confident teenage driver.

"Good landings are those you walk away from. OK landings are when the aircraft can fly again" isn't this part of the quote backwards?  OK landings have survivors, among their numbers including the plane for good landings.

@GdD: Since Burbank is in a congested urban area, perhaps noise abatement or traffic avoidance means the pilots had to use a less than optimal descent?

And don't forget noise abatement procedures which may bar the use of thrust reversers except in an emergency. And of course descent angle. A steep descent means a harder touchdown, and may be required by terrain, buildings in the area, or again noise abatement procedures.

A good joke is one you can walk away from, an increasingly desperate task to accomplish in these times... Wonder when sense of humor is declared an endangered species.

"inoperative thrust reversers requiring them to brake much harder ..." The METHOD of braking makes no difference to the magnitude of the deceleration.

@MikeBrockington Wheel brakes seem much more prone to uneven application compared to thrust reversers thus being more likely to result in forehead bruises :P IMO.

Could be that crazy winds or wake turbulence (from another aircraft) pushed the aircraft out of the funnel and forced some maneuvering to get it back on centerline and guide slope.

I later heard that pilots do sometimes make soft landings there — but shouldn’t good landings be routine?

You are relying on "passenger comfort" as an indicator of a good landing.  This ... depends on your goals.
Remember, flying is as dangerous as it would seem at first.  What makes it the safest mode (expect perhaps post-PTC rail) is constant, unflinching attention to detail, to eliminate every X-factor, to make darn sure all of a huge list of bad things don't happen.
So what makes a good landing?
A transition from "flying" to "NOT flying" which is definite and authoritative.  It happens quickly and definitely, and is  entirely in command of the pilots, not left to wind and fate.   Now we're flying, and now we're "driving".  And we have lots of weight on the landing gear so the wheels can brake and resist side loads.
Will it feel "bumpy"? Sure, a bit... hopefully not so much that a Hard Landing Inspection is required, for the sake of airfares.
What you don't want (least, not for safety) is an indecisive floating along the runway, flying at 0 feet altitude. Makes for a really gentle touchdown, but 2 bad things are happening during that time of indecision.
First, you're "on the bubble" balanced between flying and landing, and that means external forces, like gusts or wake turbulence, will unexpectedly knock you off the bubble.  Now the pilots are "behind the curve"  - reacting to events, not authoritatively causing them. They must instantly decide "do I want to go along with this or fight this?"  This indecision adds reaction time which segues to our second problem.
Also, in that state of indecision, you are vulnerable to a headwind gust tossing you back up in the air again, and the end of the gust dropping you right back down, amplifying the pilot's own reaction of trying to descend.
Or a crosswind gust pushing you sideways off the runway.  Your wheels won't help resist this if there isn't any weight on them.
Anyway, second problem: The plane is using up runway fast.  Floaty time, or confusion time, is making the remaining runway shorter and shorter. Which then adds another point of confusion: shall we try to save this screwed-up landing and stop in the remaining distance and bet on good braking action  and thrust reversers? Or "go around": max the throttles and take to the air again? Every path here also has a path to monumentally screwing up.
See, the key to aviation safety is not to tolerate variables like this.
Generally, safety favors the go-around: punch out of there to the safety of sky, get back in the queue and do the approach again.  Managements are discouraged from judging pilots by this; it's bad for safety if pilots fear political consequences for a go-around. (because it costs some money in fuel and knock-on effects of the aircraft being delayed to the gate).
So a floaty comfortable landing on a short runway, that pilot isn't doing you any favors.
What makes a good approach?
Stabilization early.   Have everything in order, early, for a no-brainer of a landing.
Many items. Flaps correct. Speed correct for weight, altitude and temperature.  Compensated for gusts. Engine power correct.  Gear down. Autobrakes armed. Spoilers armed. Go-around armed. You are heading into the correct airport.  You are lined up on the correct runway not the taxiway. Radio on right frequency. (Air Canada, go around!).   Localizer (compass direction) and glide slope ( descent angle) dialed in.
And then, crosswinds, gust, wake turbulence, and wind shear are tossing you all over the map, and you're dipping and swaying to keep it in the funnel. This is what happens when you fly in fidgety air that large airplanes used a minute ago.  This is trying to de-stabilize your landing, and you're constantly re-stabilizing it.
The more wackadoodle the winds are, the more you'll want to make that positive, authoritative transition from "flying" to "driving" with lots of weight on the main landing gear so you don't get shoved sideways off the runway by crosswinds.

"Generally, safety favors the go-around.... Managements are discouraged from judging pilots by this...": I suppose that there a need to discourage management from judging pilots by their go-around statistics because go-arounds are costly (in terms of money, primarily).  The answer might be improved by mentioning this explicitly.

Not wanting to scare non-pilots here, but it must be kept in mind that go-arounds are not inherently safe maneuvers. So each go-around must be justified, a well trained and proficient pilot can easily make the decision whether or not to commence with landing.

@Jpe61 If anything, I would say OP is not scared enough, and does not feel like "any landing you walk away from is a good landing". Wants to fly through the air like the Red Baron without seeing any ripples in their coffee cup.

Your pilot flying simply made a really firm landing.  I'm typed on the 737 and have flown the -800 in and out of short runways (Chicago Midway and New York La Guardua).  A skilled pilot can grease it on in gusty winds and can slam it on in calm conditions.
(Don't flame me bruhs, this next bit is about copilots, but anyone - in either seat - can make a firm landing.)
During a stint overseas, I had a lot of ab-initio copilots routinely slam the hell out of the -800 on 11000 ft runways.  Most of the time, they had trouble executing the flare just before touchdown.  They were robotically following the book and had no idea how to "work that plane down from 30 feet."
Burbank may scare a lot of pilots, as may LGA and MDW.  You must fly a tight and stable approach, with no goofing around in the flare.  A lot of the captains would pull up the spoilers and force the plane down if the copilot floated past the 1000 ft marks.
(sound of touchdown) BAM!
(sound of angry captain) "Don't f*ckin' float here!"
My most recent airline used a Quick Access Recorder (QAR) to monitor pilots and punish them for exceeding a tight set of company preferences.  Hard landings over 2G increased when they started busting balls over long landings or touchdowns over 1.5G.  Boeing told the company that the G sensors were inaccurate and being QAR police would not produce better flying quality, but the authoritarian management refused to take the advice.  They still have pilots slamming the runways.
So, yes the 737-800 can be landed smoothly, but there are a lot of reasons for it not to happen, especially on shorter runways.

It is better to make a positive landing than float it. Especially on low hours.

thanks for the excellent, real-world info! great stuff !!!

Yellowknife, as an example I'm personally familiar with, has 2 runways: 16/34 is 7503 feet, 10/28 is 5001 feet. I've been a passenger in assorted models of 737 landing in all sorts of conditions on both of them and have had hard landings and soft ones, and the runway length didn't have much to do with it. I was once in the same aircraft with the same flight crew that kissed the runway in Yellowknife on 10/28, landed smoothly in Rankin Inlet (6000 feet) and then slammed into Iqaluit (8600 feet).


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What happens when a helicopter loses the engine and autorotation is not initiated?
I am thinking of a very basic main and tail rotor configuration helicopter, without any aerodynamic surfaces like vertical or horizontal stabilizer, in a steady state horizontal forward flight at cruise speed. Now the engine fails completely and no one touches the controls. How does the helicopter crash?
My thoughts:

Obviously: Rotor RPM decreases ⟶ Thrust decreases ⟶ Acceleration downwards ⟶ Sink rate
Autorotation will not happen, as collective would have to be decreased to maintain RPM and get rotor to autorotation state.
The helicopter starts spinning as the tail rotor still produces lift, but there is no torque by the engine which would have to be compensated (however, the airflow around the fuselage stabilizes the direction?)

Welcome to Aviation SE, Aron!  A good first question... no obvious answers online, not even NTSB reports.  My guess: sink rate increases so quickly that nothing else matters before impact, unless you start much higher than helis usually fly.  If air drag or mechanical drag slows the rotor enough, it may eventually tumble randomly.

If the engine fails, will the rotor be "free running", or will it be blocked? And how can the tail rotor produce "lift"?

If you don't lower collective, the blades slow down and stall and the machine becomes a falling rock, with the rotor eventually slowing to a stop if there is enough time to ground contact. Yaw will depend on tail rotor pitch, and how fast it is spinning, with maybe a bit of yawing moment from bearing drag of the rotor mast between the sprag clutch and transmission housing, to the extent the rotor is still turning. Otherwise you're just a falling object.
If you do lower collective but do nothing to maintain forward airspeed, it should autorotate vertically. You'll hit the ground pretty hard, probably break your back, but may live.
There have been autogryos with special landing gear that can absorb high descent rates, and these can autorotate vertically to land.

If high enough, would the rotor eventually rotate in the opposite direction, eventually doing what an auto-rotate setting would cause (earlier and in the other direction)?

@U.Windl Helicopter blades are airfoils with a round leading edge and a sharp trailing edge. The trailing edge cannot guide a laminar airflow around the top of the blade, and the round edge cannot completely keep from doing the same thing, inducing a stall somewhere on the top of the blade, no matter what. As such, airfoils, and hence Helicopter blades, simply have no appreciable performance when moving in reverse.

Curious, which helicopter has this high impact landing gears you mentioned?

@Gabe Not helicopter, autogyro. A guy designed a very long stroke landing gear for a small open single seat gyrocopter. He could stop it in flight and descend straight to the ground and the gear could soak up the touchdown. It didn't catch on and you don't see it around much (may have been just too tippy on the ground to be practical). Root around on youtube and you should be able to find a vid.

Could yaw inputs affect the speed of a vertical descent?  What kind of yaw input would minimize it?  I'd expect a situation similar to a dome parachute, where there's an optimal amount of air to allow through the middle to ensure the parachute doesn't push down too much air below it.

Once the main rotor stalls, you have effectively no yaw control.  The tail rotor is geared to a higher RPM than the main rotor, but still has to be turning at a significant rate to produce any lift force (in the yaw direction) with its blades.

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Carrying passengers under part 91
I’m a commercial single pilot. I’m current right now but haven’t done 3 t/o & ldgs in preceding 90days.
In 2 weeks, I’m planning on a trip and we will have a hour flight with a local instructor.
Should I get 3 t/o & ldgs done before the trip? Please give me an advice on logging

Are you asking if it's allowed to have a passenger in the back seat while you get one (or more) takeoffs & landings under the supervision of the instructor?  The exact scenario you're asking about isn't clear.

In the U.S., the required three takeoffs and landings are required to act as the legal pilot in command with passengers on board.
If you are not current, you cannot take passengers with you to get current.  There are options.

Fly solo and get current so that you can take passengers
Fly with an instructor only and get current.  Neither the pilot nor the instructor needs to have takeoff and landing currency in this situation.
Fly with an instructor with passengers in the back.  The instructor must agree to be the legal pilot in command and have the required takeoff and landings.

§ 61.57 Recent flight experience: Pilot in command.

(a) General experience.

(1) Except as provided in paragraph (e) of this section, no person may act as a pilot in command of an aircraft carrying passengers or of an aircraft certificated for more than one pilot flight crewmember unless that person has made at least three takeoffs and three landings within the preceding 90 days, and -

(i) The person acted as the sole manipulator of the flight controls; and
(ii) The required takeoffs and landings were performed in an aircraft of the same category, class, and type (if a type rating is required), and, if the aircraft to be flown is an airplane with a tailwheel, the takeoffs and landings must have been made to a full stop in an airplane with a tailwheel.

The FAA legal department reasoning for the second bullet point is that there is no passenger on board the airplane. Both the pilot (being trained) and the CFI are required crew members.
https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/Data/interps/2014/Schaffner_2014_Legal_Interpretation.pdf
I also agree that if the CFI is the legal pilot in command, he will need to have a valid medical certificate.

The answer depends on who “we” is.
Landing currency only applies to carrying passengers; required crew members (such as you and your instructor) do not count as passengers.  This means it would be legal to have a lesson when neither student nor instructor is current, though that’s probably a bad idea.
If you want to carry an actual passenger, then whoever is acting as PIC must be current.  It is highly likely that the instructor will be, but I would confirm with them ahead of time since it’s unusual for the instructor to act as PIC on lessons with a rated pilot.  Also confirm that they’re okay with a passenger, since that is also unusual and raises an obvious question of whether this is a legitimate lesson or an illegal charter flight.
Oddly, which of you acts as PIC has no effect on logging under FAA rules since an instructor can always log PIC and a rated student can log PIC whether acting as PIC or being the Sole Manipulator of Controls.

“…it would be legal to have a lesson when neither student nor instructor is current, though that’s probably a bad idea.”  No worse of an idea than both flying solo to get current.

@757toga The FAA does not fully agree with the NTSB; according to an ASI, they believe there is a presumption that the CFI is acting as PIC for an instructional flight, which other regs have defined as training toward a rating.  That’s why there’s a reg now explicitly letting CFIs always log PIC; if they were always acting as PIC, that wouldn’t be needed.

from my understanding, CFI's can log PIC was written into the regs to help attract and retain CFI talent.  Without it, the CFIs could not log PIC time and future employers would question their low PIC times.  The FAA instituted that rule so aspiring professional pilots could instruct for a bit and use that time to go on to the next job.  Without that rule, there wouldn't be as many CFIs to train the next generation.  In some regards, it may have brought the quality of instruction down a bit.

I also know of the reasoning the FAA uses to violate not PIC pilots / instructors.  The thought process is whoever has the most experience as well as the legal pilot in command will be violated.  The pilot with more experience should inform the other pilots of the potential danger in what they are doing.  Hence, the CFI being violated for a hard landing performed by their less experience private or commercial pilot student.  It also goes with two pilot crews where the more experienced pilot is not the captain.

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Jettison and Dump fuel - Do they have separate meanings?
According to the definitions, is there any difference between Jettison and Dump fuel?  Does one of them use a different procedure, or they are simply synonyms?

"jettison" you mean?

In the context of fuel, jettisoning fuel would imply that the fuel is in a container that can be detached while in flight. In the linked image, the large tank below the main body is an external fuel pod that can be jettisoned. Jet with external fuel tank I believe in WWII that P51s would have drop tanks for escorts that would be jettisoned once an enemy encounter was expected.
Dumping fuel would simply mean that there is a type of valve located on the fuel tanks that can be actuated in some way such that the fuel can freely exit. Jetliner dumping fuel
To say differently, if one was holding water in a bucket, pouring the water on the ground would be 'dumping fuel' and simply dropping the bucket would be 'jettisoning fuel'
Reviewing the wikipedia article on Fuel Dumping, the author(s) make no such distinction between dumping vs jettisoning and seem to use jettison solely.
In the context of ATP, after reading the wikipedia article on fuel dumping, I believe that they are considered synonyms. I believe a non-pilot would go more along with my definition however.
I answered in this manner as I believe that the OP is not a native English speaker and that clarification was needed as to distinguishing the two as they seem very similar. I am a native English speaker, but lay no claim to being a master of it.

Jettison means to throw or drop (something) from an aircraft. Dumping fuel means the fuel (alone) is jettisoned. One can jettison any item (removable in flight) from the aircraft.
For example, a combat aircraft jettisons its fuel tank, while a civil airliner dumps the fuel, not the tank.

Jettison means to 'throw something overboard', it isn't limited to fuel. To take a practical example, if I experienced engine failure in a light aircraft and had to make an emergency landing (especially a ditching), I might jettison any large or heavy items in the cabin to avoid them causing injuries during the landing. This appears in the ditching checklist for the C172, for example (my emphasis):

DITCHING

Radio -- TRANSMIT MAYDAY on 121.5 MHz, giving location and intentions and SQUAWK 7700.
Heavy Objects (in baggage area) -- SECURE OR JETTISON (if possible).

Therefore, dumping fuel is one specific type of jettisoning where only the fuel is jettisoned.

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What are the SOPs following an overweight landing?
Modern commercial aircraft are certified to be able to land at any weight providing that the runway is long enough.  The "overweight landing" QRH procedures for the 734 and A320 are correspondingly quite short, referencing mainly the approach reference and target speeds and adjusting for wet/contaminated surfaces.
This video (which by the way, deserves repeated watching - there is just so much to look at, e.g. the vortexes high above the wings and from the flap outboard trailing edges) unfortunately is clipped from shortly after the emergency response crews arrive until the aircraft vacates under its own power.
This made me curious as to how the decision to taxi clear is made.  Are there QRH/SOPs for this, either the operator or the airport?
My guess is that the fire fighters will check the bogies for any sign of fire and, coupled with the crew checking EICAS and the wheel and tyre temperatures and pressures, make a joint agreement that it is safe to vacate.
Following vacating, would the QRH/SOP be to stop when clear and allow extra time for cooling (which could be a long time) or taxi to the stand?
Wouldn't taxiing to the stand increase the risk of a fire starting even if there was none after it had stopped on the runway?
And that video, oh, watch it again.

Flight number in the video is wrong, it is TOM6555, here are some articles on what happened. Articles say that the pilot was having an issue with the flaps (but are of course wrong about what flaps do).

@RonBeyer: Mainstream media are notoriously wrong about aviation incidents. For verified information, go to The Aviation Herald:
Incident: Thomson B738 at Cardiff on Sep 19th 2015, flaps up landing

The answer very much depends on the operator and type of aircraft (or rather its manufacturer).
First of all, overweight landing would normally be performed for the following scenarios:

Medical emergency on board that requires prompt medical attention
Any fire/smoke that cannot be extinguished/ventilated
QRH dictating "Land at the nearest suitable airport"
Any situation where safety might become further degraded by prolonged flight

It's hard to list all the situations here, and most of the time the Captain will have to make a decision whether to do an overweight landing or not.
Having made the decision to perform an overweight landing, the crew needs to take into consideration the following items:

Runway length available - landing at significantly higher weight than MLW increases landing distance substantially
Missed approach climb gradient - this might be a problem at airports with high terrain nearby
Reduced landing flap - normal landing flap speeds at higher than MLW might be too close to Vfe (maximum speed for the selected flap setting)
Required brake cooling - if heavy braking is required to stop within the required landing distance available, then it's best to vacate the runway and have fire services check the brakes and tires. Fuse plugs might have melted and tires might have deflated. Subject to all clear from the fire services, aircraft might taxi to the stand, have the chocks set and wait there with brake released for a while.
Overweight autoland is normally not recommended, as the autopilots are not certified for this.

After an overweight landing, an inspection is required by engineers to make sure there are is no structural damage on the aircraft, particularly its landing gear and the skin. According to my knowledge, the overweight landing inspection is just a bit more thorough walkaround by the engineer, provided the overweight landing wasn't a hard landing. If it was a hard and and overweight landing combined, the aircraft is likely grounded for a while, as more detailed inspection is required.
All Part 25 (FAR 25, CS 25) aircraft are certified to be able to land with sink rate of 600 ft/min (10 ft/s) at MLW and 360 ft/min (6ft/s) at MTOW, so unless you do a particularly poor landing when overweight, there should be no structural damage, even when landing straight away after a takeoff at MTOW.

The whole aircraft would be checked for structural damage, and the runway would be inspected as well.

Answers here are better received if you can provide some source for what you're saying -- either personal experience or a reference to something authoritative.  In the case of this answer, the "runway would be inspected" part of it is more wrong than right.  In many cases, you could land substantially overweight and if there was no declaration of an emergency, the airport would neither know nor care that you did.  (The runway can't tell what the max certified landing weight of your aircraft is.)


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who determines and at what point how a (larger) airport is configured for operations for the day?
This question is mostly in the context of commercial / scheduled as opposed to GA or other traffic.
It's pretty apparent that "turning around" an airport in the sense of which runways are being used is quite disruptive.  If you're somewhere like KDFW, where you have six north-south runways there are possibly dozens (maybe 100+?) aircraft and all the tower/approach/departure ATC folks that are all sort of assuming one of those directions and planning accordingly.  My local airport (KRDU) is not remotely close to as busy as a really big airport and has two major parallel runways.  Changing the direction of arrivals and departures here is still pretty disruptive.
Obviously if you have some massive wind shift in the middle of the day you're not going to have any choice in the matter, no one wants (or should, or is allowed) to take off with a 25kt tailwind.  But on a lot of days, the winds are either light and variable or "light and crosswind" enough that from a purely aeronautical standpoint it might not matter.
In these scenarios (and continuing to use my beloved KRDU), what is the mechanism that people use to figure out whether you're going to be using 5L/5R or 23L/23R?
I can see LOTS of stakeholders here: the local airport, all the airlines, both the local and the larger ATC who has to sequence in arrivals and departures, people on the ground who don't like having their houses flown over, and on and on.  But I have to assume there is SOME group of people who at SOME point sit together either literally or figuratively and say "OK, we're going to run runway 23 today, or at least we're going to start with runway 23 and if the winds get beyond [insert some threshold here] we'll flip the field."
Assuming that's reasonably close to correct... who all is part of that group of people?  Do they meet in a secret chamber at 0300 each morning to decide this?  Do they flip a very fancy aviation-runway-direction coin?  Is there some Official Meteorologist Master that provides the official forecast on which these incredibly important decisions are based???

In the usa, the 7110.65 3–5–1 is very very clear:

a. The ATCT supervisor/controller-in-charge (CIC) determines which runway/s are designated RUNWAY IN USE/ACTIVE RUNWAY/DUTY RUNWAY.
b. Assign the runway/s most nearly aligned with the wind when 5 knots or more, or the “calm wind” runway when less than 5 knots unless:

Use of another runway is operationally advantageous.
A Runway Use Program is in effect.

c. Tailwind and crosswind considerations take precedence over delay/capacity considerations, and noise abatement operations/procedures/agreements.

The wording in the paragraph was changed in the 7110.65Z CHG 3 effective 11/3/2022 (see page BG-6), incorporating N JO 7110.783 which had gone into effect that summer. This was the result of ATSAP Corrective Action Request 2012-009 and NTSB recommendation A-10-109:

Require air traffic control towers to locally develop and implement written runway selection programs that proactively consider current and developing wind conditions and include clearly defined crosswind components, including wind gusts, when considering operational advantage with respect to runway selection.

(Note how even though A-10-109 is specifically cited in the change's background information, the requirement for facilities to define numerical crosswind/gust components did not make it in...)
So the current supervisor in the tower cab—who may or may not be also working a control position at the time—is the final arbiter of which runway should be designated the main runway in use. Pilots are allowed to request the use of other runways and may be accommodated "as soon as is operationally practicable."
At larger airports the tower supervisor can expect much more pushback from the various groups you mentioned, should they choose to change the runway configuration sooner or more often than those groups think is necessary. For example: the airlines know what configuration is most efficient, and will want to use that as long as possible; as a general rule of thumb modern airliners can accept up to a 5-10 knot tailwind if absolutely necessary. The TRACON, meanwhile, will prefer to keep running aircraft in on the original configuration if there is a heavy arrival bank—they would much rather switch when traffic is light. For these reasons, runway changes often lag behind a change in the weather. But different facilities, and different supervisors, will have their own culture and opinion and way of doing things.
There is often a daily telecon among ATC facilities and the airlines for large airports/metroplex areas, and possible configuration changes will be discussed. There are also national telecons throughout the day. But the runway configuration can be changed at any time as weather develops; there is no rule that says "whichever runway is in use at 6AM will remain in use until 6AM tomorrow."

When the wind is light, as 3–5–1b specifies, the supervisor will (generally) select the "calm wind" runway. This is a runway that has been designated in the local facility SOP. There can be various justifications for that designation, but the point is to have something which can be referenced as a standard—that way nobody has to flip a coin when there's a direct crosswind at three knots, for example. Or there may be a defined Runway Use Program—for example at LAX they will generally use opposite-direction operations on the midnight shift so that aircraft are both arriving and departing over the water instead of over the city.

Excellent answer as usual. I would add that although the book says it is the tower supervisor/CIC that makes the decision, at large uperations like KCLT or KATL there is a collaborative discussion between the tower, TRACON, the ARTCC(s), and the users about runway changes. It really is cool to see TRACONs that really know how to swap directions well. The new downwinds will be occupied with aircraft as the last departures from the old operation become airborne. Is there a delay? Yes, but less than you'd think.

randomhead's answer likely applies to most of the airports in the world, but as an example of a somewhat different approach, take EHAM (Amsterdam Schiphol). As a fairly busy airport located right in one of the most densely populated areas of Europe, it is an endless source of noise complaints by the people living in the vicinity, so the airport does not have complete freedom in deciding which runways to use when. Instead, the Government of the Netherlands (its Ministry of Infrastructure) has issued a regulation (IENW/BSK-2023/13636, in Dutch only) defining a strict priority system for allowed runway combinations.
It basically boils down to a set of tables like this one (informal translation mine):
Conditions                | Priority |       Arrivals       |     Departures      |
                          |          |  Primary | Secondary | Primary | Secondary |
-----------------------------------------------------------------------------------
Good visibility, daylight |     1    | RWY 06   | RWY 36R   | RWY 36L | RWY 36C   |
                          |     2    | RWY 18R  | RWY 18C   | RWY 24  | RWY 18L   |
                          |     3    | RWY 06   | RWY 09    | RWY 09  | RWY 36L   |
                          |     4    | RWY 27   | RWY 18R   | RWY 24  | RWY 18L   |
-----------------------------------------------------------------------------------
Good visibility           |     5a   | RWY 36R  | RWY 36C   | RWY 36L | RWY 36C   |
                          |     5b   | RWY 18R  | RWY 18C   | RWY 18L | RWY 18C   |
-----------------------------------------------------------------------------------
Marginal visibility       |     6a   | RWY 36R  | RWY 36C   | RWY 36L | RWY 09    |
                          |     6b   | RWY 18R  | RWY 18C   | RWY 18L | RWY 24    |

The regulation also defines rules for when the secondary runways can be activated (basically, when the capacity of the primary is exhausted in a given period), and when a lower-priority combination can be selected (>15 knots crosswind or >0 knots tailwind on a primary, >20 knots crosswind or >0 knots tailwind on a secondary).
Normally, ATC thus just picks the first usable combination in the table. There are similar tables for night operations and for situations where one of the runways is out of service. The schedule of runway use is normally published roughly half a day in advance based on expected traffic and weather, but exceptional changes can of course be made by ATC when necessary.
So even if the wind was perfect to use 27 for departures and the airlines really wanted that as well, the ATC can't legally do that. That's because arrivals would have to use 24, and its approach path goes right across all of central Amsterdam, mightily annoying a million of voters.

Kinda sad that voters get to decide how safe your takeoff/landing is. Happened at a small GA airport near me, too. They really wanted (and needed) a crosswind runway. Had plenty of land to build it, but the voters shut it down because they moved in next to an airport (well after it had been built) and complained about the noise. :(

@FreeMan Well, I would hope that the 15 knots crosswind threshold for switching to a different runway combination was chosen to be more than safe enough. I mean, many airport only have one or two runways and also somehow manage to work safely. Schiphol has five main runways (plus one shorter one for GA and as a backup), so restricting the choice a bit is hopefully only operationally inconvenient, not unsafe.


text:
What aircraft was the Olmstead High Efficiency Propeller for? When and how was it used?
The AFResearchLab video AFRL Tech Museum Series: Propeller Development begins with the narrator standing in front of four propellers in a sequence transitioning from wood to metal, and what stands out to me at least is a very unusually-shaped two-blade propeller whose blades are extremely wide and thick near the base and taper to a point like a short, squat twisted candle flame.
There is a sign next to it that says something like (just guessing):

Olmstead High Efficiency Propeller
This unusual propeller was designed in 1918 by Olmstead Laboratories and tested at McCormick Field...

I assume that the designer is probably Aeronautical Engineer Charles M. Olmsted (1881-1948).
Here are several other photos of similar propellers:

https://airandspace.si.edu/collection-objects/olmsted-propeller-fixed-pitch-two-blade-wood/nasm_A19690140000
https://airandspace.si.edu/collection-objects/olmsted-pusher-propeller-model-elp-fixed-pitch-two-blade-wood/nasm_A19310019000

Question: It's certainly beautiful to look at, but what aircraft was the Olmstead High Efficiency Propeller for? When and how was it used?

The "Olmstead High Efficiency Propeller" is the widest one, 2nd from the right:

It was probably tested on something like a Jenny and found to be not so efficient in actual application, and put away.  It looks like an attempt at a blade that operates at a constant Reynolds number across its span.

This propeller shape is for general purpose use. It attempts to optimize the lift distribution across the prop to maximize efficiency by using chord along with other design parameters. Unfamiliarity with construction and installation combines with only a slight improvement in efficiency has limited its popularity. Modern takes on this shape were seen in Paul Lipps' winning Reno racer

and in the propulsor prop of the Carter Copter.

Propellers are often described as wings. Efficient wings have an elliptical (or close) lift distribution to minimize drag. This approach is extended to props and results in a wing like planform, but a prop flies very differently than a wing. Relative airspeed at a prop tip is twice that at midspan and four times that at quarterspan, where on a wing all the airflow is essentially the same speed.
Since lift is a function of speed squared the required coefficient of lift at the root is two orders of magnitude higher than at the tip. Shown is an example of a 72in dia prop with m0.8 at the tip.

The three main design parameters used to get the required CL are chord, camber and angle of attack (or twist). Most propellers designers can get the desired lift distribution using camber and twist, and forego the complications of using large changes in chord.

If you zoom in on the video still, you can read it: "...was designed 1918 by Olmstead Laboratories ... for possible use on the Loening M-8 aircraft...".

Thanks to @Walen for this better resolution:

Full resolution image with clearly visible text at https://www.flickr.com/photos/156402454@N04/47673679042 confirms this.

This could have been a good 'before' and 'after' of that "zoom enhance"-feature they always use in the movies :)


text:
What are the separation requirements on the ground?
What are the FAA regulations for aircraft separation on the ground, outside of active runways?

https://aviation.stackexchange.com/questions/2806/how-much-is-the-minimum-safe-distance-between-two-planes-in-flight

Except in relation to active runways, the FAA defines no specific minimum separation between aircraft operating on the ground.
JO 7110.65 paragraph 2–1–1 states that "the primary purpose of the ATC system is to prevent a collision involving aircraft operating in the system." So ATC must issue instructions that ensure aircraft do not "trade paint" (collide) with another aircraft, a vehicle, a structure, a pedestrian, etc.
Furthermore, the ATC system is intended to "[provide] a safe, orderly, and expeditious flow of air traffic." So ATC instructions should also ensure that aircraft do not end up pointing nose-to-nose on the same piece of pavement with no way to turn around and no room to pass by each other: a "Golden Towbar Award" moment.
For pilots, 14 CFR 91.111(a) requires that "no person may operate an aircraft so close to another aircraft as to create a collision hazard."


text:
Why would one wing stall before the other?
Reading about the Douglas C-133, I see this:

The second issue discovered was stall characteristics gave very little
  warning to the crew. The left wing was found to stall before the right
  wing. The fix was simple, a small strip of metal was attached to the
  right wing causing it to stall at the same time the left wing would
  stall.

The issue affects the whole type. What would cause one wing to reliably stall before the other?

It has to have something to do with the fact that all four engines turn in the same direction

Interestingly enough, this article (which Wikipedia cites), says that "Cameras... clearly showed the right wing stalling before the left—in fact, the left wing usually didn’t stall at all" http://www.airspacemag.com/military-aviation/the-curse-of-the-cargomaster-2241392/#hZBCSB8ibv5MryLM.99

In general, aircraft aerodynamics can be affected from manufacturing tolerances or consistent asymmetries.
The reason can be due to any asymmetry in the wings or fuselage shapes, or wing-to-fuselage fixtures.

Are you suggesting their jigs were crooked?

Exactly. The consistent asymmetry might be the result of a fixture being asymmetric, or a mould or template.

In a non-design respect, sharp yawing pressure during a stall can result in one wing stalling more than the other. That causes a spin. If you're looking for a design that can do it, a square of metal near the top of the camber can increase the turbulent airflow to match or exceed the other wings'. Any difference of the wing shape, airfoil size, or airfoil shape between one wing and the other can also result in one wing stalling more easily than the other.

For any design, a skidded turn stall is an example of one wing stalling before the other.  This would be characteristic of a handling error rather than a design issue.
In an aircraft not in balanced flight, particularly in a skid (nose inside the radius of turn) the fuselage will somewhat mask the airflow on one wing causing it to stall before the other.
We used to teach these to instructors at altitude when I was a standardization pilot in the Navy, for the T-34C, to illustrate the disorienting rolling moment that occurs when one wing stalls.
Close to the ground this kind of stall could be lethal
(Please view the whole video, the narration by the instructor is very good).
The video example shows a rapid loss of about 700' with an instructor knowing it was coming.  He calls it a "base to final skidded stall" that he describes as causing "an incipient spin."
This is a form of a cross control departure.  A college classmate of mine died in a cross control departure in the landing pattern, in a T-2 over 30 years ago during flight training.
(I just discovered that the Navy now prohibits cross control departure training in the T-45C jet trainer which IIRC was part of the OCF syllabus in the T-2. I also found it missing from the T-6B syllabus, which I must say surprises me).

Sure, but if you did the maneuver the other way then the other wing would stall first, correct? The C133 writeup implies that the problem was systemic for the whole fleet.

I'm curious about the C-133 case in particular. I'm only assuming it's a design issue if the fix was a piece of hardware.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why was the An-225 built when the VM-T Atlant already did the job?
The Soviet Buran program decided to use a carrier aircraft to move their orbiter and Energia tanks similarly to the American Shuttle Carrier Aircraft. However, the Soviet Union built two types to do the job.
The famous An-225 project was started early, but they also decided to build a backup aircraft based on a Myasishchev 3M bomber. This aircraft, the VM-T Atlant, was finished much earlier. Since the An-225 wasn't ready yet the Atlant played delivery truck for the initial and only Buran launch.
Of course the An-225 is much more capable than the Atlant, but if the Atlant can do the job why go through the effort and expense of developing a second massive airplane?

Further reading on the VM-T Atlant.

Those are some huge vstabs!!

As you can see in the second photo, the VM-T was unable to lift a fully-assembled Buran. The VM-T max payload is 50 tonnes, a Buran weighs on the order of 80 tonnes.
So for the VM-T flights they had to strip the Buran to get the weight down, including removing the tailplane.
The An-225 could lift a fully-assembled Buran, but was not ready in time, so they used the VM-T as an interim solution.

Soviet ingenuity


text:
Why would a propeller have blades of different lengths?
The Schempp-Hirth Duo Discus T powered glider has a five-bladed propeller, with blades of different lengths.
What is the purpose of such an arrangement?

They look to me of the same length (https://www.google.com/url?sa=i&source=imgres&cd=&cad=rja&uact=8&ved=2ahUKEwi7qN7Us5LjAhUEWxoKHczYDq4QjRx6BAgBEAU&url=http%3A%2F%2Fwww.wildbergair.com%2Fphotos%2Fflightimpressions%2Fgliderfi%2F20100709%2F20100709_01.htm&psig=AOvVaw2edGqsRiPjmhd7LRGqr9cS&ust=1562025252535319)

I tried to find a conclusive source for the differing length, and funnily enough, only ran across it in this accident report: https://assets.publishing.service.gov.uk/media/5714f22aed915d1601000000/Schempp-Hirth_Duo_Discus_T_G-SAXT_05-16.pdf
It seems the propeller hub had some fatigue issues which were not detectable with the suggested inspection procedures. The manufacturer does not state anything about different lengths on the open portion of their website.

Asymmetrical fan blades are nothing new. Automakers have used them for decades to reduce noise from radiator fans.  They, however, are the same diameter because they are ducted.

You are right.  They are unequal length blades.  Schempp refers to the propeller as being "noise optimized" and you can see that the blades are free to flap (forward) like helicopter blades (which flap up) so that the asymmetric thrust axis from having the propeller disc offset from the rotational axis doesn't place bending loads on the hub.  And obviously, the blades will be mass balanced to place the center of gravity at the hub axis.
I'll go out on a limb here and say that some genius figured out a way to reduce the horrific noise level caused by such a small propeller by having the blade tips each run in a slightly different path of rotation.  Brilliant.

And here it is in action where the eccentric blade disc can be clearly seen.

May have something to do with unequal blades cancelling, rather than reinforcing, sound waves.  Great answer and pic.

They look like they are also unevenly spaced around the hub, which is a known technique to reduce noise. https://escholarship.org/content/qt9q75v9t9/qt9q75v9t9.pdf

@MikeY now that you mention it, yeah it's obvious when you look carefully.

Excerpt from the conclusions of that paper: "The results showed significant increases in total acoustic intensity for propellers with uneven blade spacing, with very high concentrations of energy in the 0.5BPF tone. [...] This is beneficial for noise metrics such as flyover noise, as noise at low polar angles are “weighted” less than noise near the plane of the propeller." So it is more noisy, but in a lower frequency which is less annoying to humans.

But does the uneven spacing and blade length affect performance or safety in any way?

@Cloud balance of the prop would be a big issue but that is what weights are for.

Noise is wasted energy, so my guess is yes, it is less efficient. Regarding safety, only as much as other unusual designs, by merit of being unusual. The early model prop hubs developed fatigue cracks that went undetected, but this seems to have been fixed.

@AEhere:  It's not that the noise is at lower frequencies, it's that the noise is "thrown" forward (along the axis) more than it would be with a symmetric design, and correspondingly "thrown" less in directions perpendicular to the propellor axis, including down towards the ground.

@Cloud I guess anybody smart enough to come up with this idea would realize you need to keep the prop balanced - hence the fact that the shorter blades are closer together than the longer ones. There will still be a rotordynamics issue because the moment of inertia about different axes will vary - but that can be designed for, otherwise 2-blade propellers and helicopter rotors would not be a thing!

@MichaelSeifert that too. I realize that the part I quoted is not the best as it touches on both topics, so reference page 39 of the report: "The shift of acoustic intensity into the low frequency 0.5BPF tone is beneficial because the human
ear is much less sensitive to low frequency sounds." and the accompanying graphs; you can clearly see additional peaks in the 1-5 kHz range.

@alephzero well, 2 blade helicopters are statistically much more dangerous than a twin jet aircraft

@Cloud only because of the "mast bumping" phenomenon.

Just a guess, but it might be an attempt to reduce the chance of resonance at the blade frequency.
The tail will receive pressure pulses from each blade, but they won't be evenly spaced.
A motor like this is designed to be started and stopped during flight, and will this accelerate and decelerate through a range of speeds. Somewhere in the range between 0 and full power may be the resonant frequency of some part of the tail. When the motor hits that frequency, the tail could shake violently.
By spacing out the pressure pulses from the prop, there won't be any speed where they all resonate with the tail.

Cars (and laptops, apparently) reduce objectionable noise similarly: https://mechanics.stackexchange.com/questions/24327/why-are-the-blades-of-car-radiator-fans-unevenly-spaced

The engine on the Discus cannot be throttled in a conventional sense, it only has on and off modes.

@AEhere, but as it goes from Off to On and back, it goes through all the speeds in between. The control might be binary but the engine rpm is still analogue.

Yes, of course, I was merely pointing out that the control is a bit unusual. Also, the pilot must decompress the engine for it to start, so there's that.


text:
Can longer paths consume less fuel than the (shorter) great circle path?
Is the most fuel-efficient route between points A and B always along a great circle ?
Although somewhat counter-intuitive, this comment on an aviation blog seems to suggest that longer paths (in terms of geographic distance covered) can save fuel:

Oceanic Flights these days are not restricted to the old “defined
  great circle routes”. Modern computerised flight planning systems
  definately take weather into account, and go searching for the best
  winds (tailwinds) or least adverse (headwinds). Particularly in the
  case of westbound flights across the Indian Ocean, where headwinds are
  often ferocious, the tracks actually flight planned, and actually
  flown, are often a long way off “great circle” routes.

The quoted text seems to be a perfectly adequate (and obvious!) answer.

Related: Why are westbound transatlantic routes located hundreds of km away from eastbound routes?. And the reason being the presence of jet streams, this question is also related: Are jet streams a net benefit in time and fuel savings?

On a flat plane, the shortest distance between any two points is a straight line, right? So if you were at A and wanted to get to B in as few steps as possible, you would walk a straight line.
But what if there's a moving sidewalk along that route, going the wrong way? Now is the path of fewest steps still a straight line? Probably not: by adding a few steps at the start and end, you can bypass the moving sidewalk and avoid the extra steps it would force you to take. The moving sidewalk essentially added more ground, so that the shortest path via a tape measure is not the shortest path by steps taken.
The idea is the same with headwinds. The shortest path as measured by the ground is not the same as the amount of air you fly though, because the headwind effectively add more air. Since the engine is working on the surrounding air, not the ground under you, the fuel consumption is based on how much air you fly through -- not how much ground that happens to cover.

great analogy !

That depends how you define longer in aviation. As you can look at it geographically or on the clock.
The hard answer is no, the most fuel efficient route is the one that has the aircraft in the air for the shortest amount of time. If you can take advantage of a big tail wind 20 miles south of the geographically shortest route or at a slightly higher altitude you may burn a bit of fuel getting there but on long haul flights you can make up for the burn, and even gain quite a bit by taking advantage of the winds
Similarly headwinds can really kill your efficiency even over short distances.

"Longer" is meant geographically in the question. I thought this was apparent from the context. How should I rephrase the question to make that clear ?

I think its clear, i have edited a bit to clarify my answer.

@summerrain How about "Can a longer path in terms of geographic distance covered consume less fuel than a great circle?" As Niels said, the answer is yes. Also be aware that it's possible that the route that results in the smallest fuel burn is not the necessarily the route that takes the least time. That's  true more often than not, but if to get the most favorable winds you had to stay low, you might burn more fuel.

@Terry never mind! You answered it!

Yes, the shortest route by distance (great circle route) is very often not the most fuel efficient route to take.
An aircraft at a given speed, altitude and weight will burn a predictable amount of fuel per hour, regardless of the distance traveled. It follows that the less time in the air, the less fuel is burned (generally speaking, and if flying economically). If there is a tailwind, the aircraft will travel faster without using any extra fuel, needing less time to go from point A to B, ergo less fuel consumed. So a pilot or dispatcher will review the forecast winds, and if they are strong enough, will detour from the direct great circle route to take advantage of them (or get out of their way if it is a headwind).
Anecdotally, I would predict that almost every single long haul flight flying east/westwards will deviate from the GC route due to winds (as opposed to other route changes like airspace restrictions).

It may be nit picking, but the statement, "An aircraft at a given speed will burn a predictable amount of fuel per hour" might be more strictly stated "An aircraft at a given speed, altitude, and weight will burn a predictable amount of fuel per hour.

@Terry nit picking or not, it could have left the impression that flight planning is anything but complex, so I have edited that in! Thanks

A good example of this is the new software QANTAS has rolled out. https://www.smh.com.au/business/companies/how-subtle-changes-to-flight-paths-will-save-qantas-millions-a-year-20181126-p50icy.html

Scheduled operators might choose to fly the great circle route anyway, but at a different power level, to maintain the same arrival time.  Arriving late could mean lots of missed connections, arriving early may result in not having a gate available, and either could conflict with noise abatement rules.

You begin with a great circle route as the default. Then you adjust it to account for the presence of either favorable or unfavorable winds.

So your answer is YES ?

yes. it depends on the winds present along the flight path.

You need also to take into account the ETOPS capabilities. ETOPS allows to navigate remote from diversion airports.


text:
Why is there a cutout in the pilot seat (looking as if it had separate "legs")?
Why is there a gap in the seat (between the legs)?
What function does it have?

I can't speak to all aircraft, but on the 727-100 and 747-100/200 aircraft I flew, the seat cutout accommodated the strap on 5-point seat belt systems that went down between your legs. See
https://pitstopusa.com/c-134853-safety-equipment-seat-belts-harnesses-cam-lock-restraint-systems-5-point-camlock-restraints.html.
I don't recall for certain whether the control yoke could be brought all the way back into the cutout, but I don't think it could. I may be misremembering though.
As I remember, the camlock into which the other 4 straps (left and right across the seat and left and right shoulders) was permanently attached to the strap going down between the legs. On sitting in the seat, you reached down and pulled that strap up with the camlock on the end and then put the other 4 straps into their slots.
I greatly disliked the shoulder restraints, so once we reached cruise climb, I would rotate the camlock while holding the left and right seats straps in. This would release the shoulder straps.

The seats are adjustable both forward and up, aren't they? Then the control column might reach into the cut-out for a pilot who is really short and has the seat all the way forward even if it does not for you. For what it's worth, Airbus aircraft with side-stick have seats with no cut out and the crotch strap attached in the middle.

@JanHudec I'm short so my standard seat position was all the way forward, all the way up, and the rudder pedals (also adjustable) all the way aft. As I remember (accurately?), the yoke came an inch or two into the cutout. So, perhaps the primary reason for the cutout was for the yoke, but the designers elected to simply continue the cutout to accommodate the crotch restraint.


text:
Why are fuel tanks located in wings?
Passenger aircraft have fuel tanks in the wings. Why?
What are the advantages and disadvantages of this location ?
examples of disadvantages I would suspect:

added weight increases the structural load applied to the wings
different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span
higher risk of catastrophic damage to wings in case of in-flight fuel ignition
higher risk of fire when lightning strikes a wing

Keeping fuel in the wings actually helps to maintain their structural integrity, see https://aviation.stackexchange.com/questions/42613/why-are-fuel-tanks-in-the-wings-filled-first-and-why-are-they-used-last?rq=1

On the ground, the aircraft rests on the wheels. In flight, it rests on the wings - so weight in the wings reduces structural loading, not increases it.

@Therac: Following your explanation, I would see that it does not add structural load? But how would weight "reduce" structural load?

@summerrain By balancing out the fuselage, which is a big central load. See A340 vs A330. Of course, this implies total weight is equal or within some limit.

@summerrain it doesn’t reduce it per se, it reduces the need for an even high structural (bending) load that would come from storing it in the main body.

Actually as @Therac mentions both OP's points one and two of supposed disadvantages in reality are advantages.

Where else would you put the fuel???   There's literally nowhere else.  I mean, you could turn the passenger compartment in to a fuel tank I guess?

BBC have an hour-long documentary interviewing people at an AirBus plant that builds such wings, which includes some discussion on why and how the wings are used to store fuel including how the fuel balancing works. Might be of interest. Official BBC link, unofficial YouTube link

@Fattie I'm pretty sure there are some people who would pay to sit on the wings. Wonder if this is a commercially viable concept...

@DoktorJ haha - Unfortunately, it's not very aerodynamically viable...

Several advantages:

Wing structures are hollow and voluminous in order to provide structural rigidity against flutter and carry flight loads.  This provides the space needed to store fuel.
On a conventional aircraft, placing fuel tanks in the wings places the fuel mass very close to, or on, the center of lift.  This dramatically reduces Cg shift during flight and reduces the size and weight of the tailplane to maintain stable flight.  It also reduces Cg shifts due to sloshing of the fuel inside the tanks, due to the limited constraints of longitudinal travel for the fuel in the tanks.
In the event of a crash landing, having the fuel in the wings keeps it away from the cabin and the occupants, reducing risks of cabin fires.
The weight of the fuel reduces the loading moment on the wing roots, reducing the weight of the structure needed to support the aircraft during flight.
Fuel stored in the wings either partially or completely eliminates fuel storage in the fuselage, leaving more room for passengers and cargo.

Disadvantages:
1) Fuel sloshing with in the tanks laterally as a result of turbulence, or uncoordinated flight, can lead to lateral weight shift and potential lateral instability.   At low quantities of fuel, and in prolonged uncoordinated flight, there is a chance that the engine can suffer fuel starvation simply because the fuel has flowed out of the sumps in the tanks These problems can be alleviated with proper fuel tank baffling and the use of feeder hoppers fed by the main tanks which the engine drinks from.
2)  On aircraft which utilize a siphon feed fuel system such as low wing aircraft, the fuel cannot be evenly siphoned from both tanks at once.   This is a particular problem in single engine aircraft, where separate fuel systems are not dedicated to one engine in particular.   In such cases the engine will feed off either the left wing tank with a right wing tank  and this is controlled by means of a fuel selector valve in the cockpit. On aircraft that do not have automatic fuel management systems, engine fuel feed must be manually selected.  Care must be taken to alternate feed from both tanks periodically to prevent a lateral imbalance and fuel quantity. Additionally this fuel tank switching schedule, if ignored long enough, can potentially lead to fuel starvation of the engine and a forced landing.  This is particularly problematic in the low wing light single engine aircraft such as the Piper PA-28 or Cirrus SR-2X, especially if the pilot has recently transitioned into this airplane after flying high wing aircraft, which use gravity feed fuel systems, and allow the engine to feed from both tanks at once.  Larger single engine aircraft like the TBM have automatic fuel tank switching systems to address this problem.   Large multi engine jet aircraft have dedicated fuel management systems which address these problems.

Possible additional: adding weight to the wings increases their inertia, reducing the influence that turbulent airflow has on them. In simple terms, if the air/wind spends all its time trying to wag the wings up and down and snap them off the plane, making the wings heavier makes them harder to wag, reducing the amount of repeated bending stress the mount points are subject to, and providing for a plane that flies more stably

Also: increasing fuel capacity in the fuselage would reduce available cargo space; the wings can't be practically used for this, but they can be used very practically for fuel because it's not constrained to a particular container height/size. It's liquid, so they can fit the cell around and between structural parts of the wing and make use of a great deal of it. Conversely, relatively few suitcases could be fit practically along the wing, probably primarily near the root, and trying to design around the necessary volume would be an engineering challenge to keep the wings structurally sound.

@GalacticCowboy not to mention, trying to balance the weight, since same-sized pieces of luggage may weigh differently.

You list advantages only. Are there also any disadvantages ?

Added more on that subject.

I see what you're saying, but there's something you're overlooking in your logic.  You're looking at an airplane sitting on the ground, where the wheels are near the fuselage and most of the wings are dead weight that creates strain on the structure.
Think about one in flight.   Now all the lift is coming from the wings, imagine the airplane suspended by a couple dozen (billion) cables spread around the wing surfaces.  Now the fuselage is dead weight and the strain in the structure is from carrying the fuselage.
So when you add weight to the wings evenly, it adds practically zero structural load for the wings. What's being lifted is inside the source of the lift. So from a structural load perspective, it's a wash: it doesn't matter.
Whereas if you add more tanks in the fuselage, that's fine on the ground, but it adds huge stresses to the wings in flight, effectively reducing practical cargo capacity.
The strain on wings from sitting on the ground is much less worrisome to designers than the strains in flight.
See also "Zero Fuel Weight".

Exactly what I was thinking, but you said it much better!

+1, for a great explanation.

"where the wheels are under the fuselage and the wings are dead weight that creates strain on the structure." One important note here: Most airplanes with tricycle landing gear actually have the main gear mounted to the wings, not the fuselage. Some very large aircraft do have an additional inboard set of main gear under the fuselage, but they usually still have outboard main gear under the wings.

For example Boeing 757, PA-28-140, Airbus A320, Boeing 747

added weight increases the structural load applied to the wings
  different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span

As a result of the effects of lift (and the deceasing need for it as the plane lightens) the reverse is actually true see here

higher risk of catastrophic damage to wings in case of in-flight fuel ignition

As opposed to higher risk of catastrophic damage to the cabin in the case of in-flight fuel ignition?
Assuming a non-explosive ignition having the fuel in the wings means you can take action to dump the fuel. If you have a fire begin in the main fuselage however you've got a higher chance of the fire incapacitating the crew before they can take steps. Or damage occuring to the avionics, the pressure cabin etc.

maybe higher risk of fire when lightning strikes a wing ?

Wing tips are one of the locations on a plane that is more prone to lightning strikes - and the potential for fuel fires is there but steps are taken to counter this and in the vast majority of cases lightning does very little damage

which are the steps taken?

Quite simply: there's a lot of empty space in those wings, and there's a lot of empty space needed for fuel.
Creating space elsewhere for fuel would make the entire aircraft larger and heavier, so makes little sense.
And it's not just the wings, many aircraft carry fuel in the vertical stabiliser as well.

In other words: Why are fuel tanks located in the wings? Because the passengers wouldn't fit in there. :)

@TannerSwett Don't give them any ideas...

@pipe https://en.wikipedia.org/wiki/Junkers_G.38 ?

@TannerSwett hmm, Ryanair wants to talk with you about that idea.

Along with the other answers, I'll point out most of the recent the cases where an aircraft fuel tank exploded, the center tank, which is in the fuselage, was implicated. There are two reasons:
First, a fuselage tank is located lower than the engines and requires pumps to raise the fuel. Electrical pump failures have caused explosions. This also means that a pump failure results in unusable fuel, whereas wing tanks can naturally feed the engines via gravity.
Second, fuselage tanks are closer to sources of heat. This was a cause of the TWA flight 800 accident, where heat from nearby air conditioning equipment lead to a flammable vapor in the fuel tanks. In contrast, wing tanks are naturally cooled by airflow and are less susceptible to forming such explosive vapors.

Not sure if you meant "that in all the cases ... it was the center tank which was implicated" (which is apparently true), but if so, you could probably make it more explicit.

@jcaron Reworded. I'm sure a wing tank has exploded at some time in the history of aviation, but certainly the center tank has been the main cause of accidents recently.

added weight increases the structural load applied to the wings

Only when the plane's on the ground. When it's in the air, it decreases the load on the wings because their lift balances the weight.

different gravitational forces and wing-bending between full and empty tanks result in repeating stresses shortening the aircraft life-span

At the rate of one cycle per flight. And the wings already go through a stress cycle once per flight (flexed down when the plane's on the ground and up when it's in the air).

higher risk of catastrophic damage to wings in case of in-flight fuel ignition

The fuel tanks catching fire in flight is catastrophic wherever you put them.

higher risk of fire when lightning strikes a wing

When did that last happen?  Wikipedia's list of plane crashes suggests LANSA flight 508 in 1971. Such incidents are now even rarer, because fuel tanks are fitted with inerting systems. This was originally recommended after the crash of Pan Am flight 214 in 1963 but it took a long time for it to actually happen.

This answer seems to imply that inerting systems have been used since sometime shortly after 1963, or at least that inerting systems are responsible for the lack of lightning-induced fire since 1971. The link you provided, however, says inerting systems were not installed for 40 years after the crash of Pan Am 214. Here's an article describing the state of the art in 2004: https://www.airspacemag.com/how-things-work/safer-fuel-tanks-5883916/

@DavidK Yes, it looks like I've exaggerated what actually happened. I'll come back and edit when I have time, or you're welcome to propose an edit yourself if you have the time. Thanks for letting me know.

@DavidK Edited.

re: "fuel tanks catching fire in flight is catastrophic wherever you put them" Although any fire is critical, not all locations are equal. If you lose a wing you go down instantly. And you would not put the fuel tank directly below the cockpit for example.

@summerrain A fuel tank right at the front of the plane would cause serious balance issues as it was drained. Do you have any evidence that fuel tank fires aren't always catastrophic?

I was talking purely from a security standpoint. Balance aside, do you think fuel tanks would be placed directly underneath the cockpit of all places? re: fuel tank fires are always catastrophic - I don't dispute that. But depending on the location you may have slightly more or less time for an immediate emergency landing.

@summerrain Do you have any examples of planes that have made successful emergency landings after fuel tank fires? It feels like your argument is completely hypothetical.

You ignored my question. And what would an example prove/disprove ?

@summerrain I already explained why there aren't fuel tanks under the cockpit. Your question amounts to "In an alternate universe, would..." and we're not in an alternate universe so there's no value in addressing your question. As for an example, you are claiming that putting fuel tanks in different places would give more time for emergency landings. I suggest to you that fuel tank fires are so catastrophic that no emergency landing is going to be possible. A great way for you to argue against that claim would be to say "But it happened on flight XYZ."

I'm simply saying that from a security standpoint alone, you wouldn't put the fuel tanks directly beneath the cockpit. That's self-evident, no? And of course depending on the location you might get a few more/less minutes (or are you expecting exactly equal amount of time?). This is difficult to prove by way of example, since every crash is different: We wouldn't even know the exact time of onset of the fire, how the fire spread, and hundreds of other parameters. I'll rest my case. You gave a very good answer and we are in general agreement, though maybe not at 100%.

Just my 2c. If the plane's fuel is "Jet Fuel", the likely hood of it ever being set alight or exploding is close to impossible as this fuel is refined kerosene and does not burn without being atomized.

@TheLegendaryCopyCoder There have been plenty of plane crashes over the years caused by exploding fuel tanks. You're forgetting that kerosene vapour can burn, too.

More weight on the wings is actually good as it makes the aircraft more balance and more resistant to unnecessary swerving during turbulence or wind current, like a person walking on tight rope carrying a horizontal rod(bar) for balancing. Check out radius of Gyration in mechanics.

There is no other reason.
The engine is attached to the wing and They are trying to design a fuel tank to fuel the engine,
There is no space on the torso. So they make a hole in the wings.

welcome to aviation.SE. Please be aware that this is an english-only website. as such, we will have to remove the part of your answer written in Korean. Moreover, please be aware that we like answers that contain a bit of detail, and possibly references.

Why are fuel tanks located in wings?

Because it is hard to fit the passengers or their baggage into the wings.


text:
How do pilots decide whether to circumnavigate bad weather?
How does an airline pilot decide in the air whether to circumnavigate or fly through bad weather visibly looming ahead and/or displayed on the weather radar ?

Related

How "bad" do you mean the weather to be, and what type of plane? I suspect you mean airliners and thunderstorms or blizzards, but please clarify if that is (or isn't) the case.

3 perfect answers so far. Therefore I have no idea, which one to mark as "correct". Any advice ? This is a perfect example of where multiple answers are and should be acceptable.

@StephenS: I clarified the pilot definition, thanks. WX is already clearly defined though, no? Either visibly looming ahead or displayed on WX radar counts.

Easy.  You don't go through, or close to, a thunderstorm cell (a red area, which is lots of water), period.  You decide on a heading change to navigate around it, request it from ATC, and if the heading you want is not forthcoming, you tell ATC you are altering heading for weather.  You are not required to endanger the aircraft to comply with an ATC clearance of any kind.  And getting close to a cell is endangering the aircraft.
Different airlines will have different policies on some of the details of getting around cells, like they have to maintain 20 miles minimum, upwind if possible.  In visual conditions where you can see the cell, same deal.  Alter heading to upwind side x miles (you don't want to fly under the anvil which is downwind, even if the air is clear in front of you, because you can run into hail falling from the anvil).
Besides thunderstorms, the other big bugaboo risk wise is icing the plane can't handle.  Same deal, you ask for higher or lower, and if icing is becoming dangerous and you need to move now, you tell them you are going higher or lower.

There are potentially as many reasonable answers to your question as there are pilots that have had to regularly pick their way through lines of thunderstorms. In that sense, the answers may be as much opinion based as factual. However, I think there's enough objectivity involved to hazard my hopefully somewhat objective opinion.
Let's say the image of the radar display you posted is what we have to work with. We're over the open ocean a long ways from any ground based radar.
Further, let's assume we have the elevation, sensitivity, and range of the radar appropriately selected, and that what we see on the display is all that we have to contend with along our straight-ahead route, at least insofar as the red areas, and that a major detour left or right to avoid the entire line would mean stopping short of our destination to refuel. Thus I'm going to pick my way through what I see ahead.
I'm looking at two potential paths if in a passenger airplane, three if in a freighter, the difference being that in a freighter I'm not concerned with passenger comfort—boxes don't complain.
Path 1 (Pax): Fly only in green and black with a short bit through the orange on a course of 122° to the green such that a left 90° will take us into the black, then work through the black until a course of 102° will take us out of the line. That's tempting, but can I trust the radar? The opening out of the line has on it's left side a rectangular end of the orange, and nature doesn't usually deal in nice rectangles. Plus the radar return for that clear way out is behind a substantial area of red, which would have attenuated the signal.
Path 2 (Pax): Same as Path 1 except stay on 122 util out. The disadvantage is once past the point where a left 90° turn would take us to the black, there's a significant amount of orange. If there are 480 people in the cabin, there's going to be projectile vomitting, and when the first person goes, the dominoes fall.
Path 3 (Freighter): Obviously a freighter can use either of the passenger paths, but without having to be concerned for passengers, instead of an initial course of 122, use 110 (saving fuel) until into the black and then work around left.
Which would I use? I think I'd go for Path 1. If the opening really isn't as displayed, my best guess is that a dash through orange is what it will be, and if there's a little red, it will be short. I don't think any plan I ever started with didn't have to be changed as the picture ahead changed.
The majority of my experience with threading my way through thunderstorm lines enroute was five years of flying the Indonesian Hajj—Jakarta to Jeddah and return. It was four hours of open water from the west end of Java to Sri Lanka, and often meant crossing the intertropical convergence zone (ITCZ). We pretty much flew a great-circle. Had to to make it without refueling, and that meant crossing the ITCZ at an acute angle rather than at 90°, and that extended our time in the zone.

thank you for this great answer. I'm sad I can only mark a single answer as correct. Yours is just as excellent.

When faced with bad weather there are several considerations.  A practical one is whether you will be able to get to your destination. That is, will the weather be worse there, or will there be conditions which prevent your arrival. For example, flying through pre-frontal precipitation (precip hereinafter) may be pale in comparison to landing with 65k gusts running perpendicular to a single runway.
So looking at the actual enroute threats, they fall into the following categories: turbulence (mountain wave, thunderstorms and other situations which pose the risk of loss of control to the aircraft, and have the risk of exceeding operational parameters of the aircraft), icing (airframe, and to a lesser extent powerplant icing and prop icing) and finally precipitation (hail, which can cause structural damage, or even heavy rain which can cause powerplant failure.  So the issues are turbulence, icing and precipitation. We will ignore other phenomena, which are much more situational, such as volcanic ash, etc.
Up until a few years ago, onboard radar was the primary method of detecting precip. The size of the dish is a factor in the performance of the radar unit, and single engine aircraft with wing mounted radar are at a disadvantage.  While onboard radar may show precipitation, it can be attenuated by near-field precip, masking heavier precip in the far-field.  Smaller units are more susceptible to this than larger units. Large aircraft tend to have large antennas and are more effective at reducing near-field attenuation. The simple strategy with radar is to avoid flying into areas which show heavy precipitation. More complex strategies look at precip patters such as hook clouds, which may indicate threats such as tornadic activity.  Heavy precip, indications of hail, hook clouds, are all predictors of turbulence, and flag areas and routes to avoid.
Today, NEXRAD pulse-Doppler radars are aggregated and processed and provide imagery via uplink to aircraft with more effective large area coverage than onboard radar. NEXRAD may be used in smaller aircraft, and may also be utilized as supplemental area coverage for situational awareness.
Most, but not all radars output three or more levels of returns. Level 1, green, is generally considered to be light precip and is normally flown through.  Level 2, yellow, corresponds to moderate precip, and will have turbulence. Level 3, red, correlates to heavy precip, and may contain thunderstorms with risk of severe turbulence and structural risk from the turbulence. Normally level 3 is avoided.
Onboard radar has a tilt capability and is able to provide some information as to what the precip levels are at different slants from the aircraft, which can then be used to help interpret activity as well as be factored into route selection.
Networked radar, such as NEXRAD, not only provide the integration of multiple stations, but can also provide post-processing to reduce artifacts and overcome single-station radar limitations.
Icing is determined by a combination of factors: freezing level and type of return.  Networked radar, utilizing post-processing, generally provides the best assessment of icing conditions. Crystalline water has different reflectivity than liquid water.  Furthermore, incorporation of aloft air temperature information refines the icing prediction.
Non-radar methods of determining turbulence are also effective at avoiding bad weather. Discharge detectors, which process RF from static discharges are effective at displaying turbulence in mountainous regions, where there is not enough moisture in the air to show clear air turbulence and mountain wave activity.  These devices have proprietary signal processing to classify and display various types of static discharges and provide pseudo range information.  These units are very effective, and when coupled with both onboard weather radar and NEXRAD provide the highest level of awareness to threats from turbulence and thunderstorm activity.
The answer the original question, circumnagivation decision making is done with risk analysis of the radar imagery information available, as well as PIREPs and ATC provided supplemental information. These are factored using the specifics of the flight, and company guidance to determine whether the flight continues, gets diverted, and how the pilots will elect to route themselves between and through threats.

Excellent answer. "company guidance to determine whether the flight continues, gets diverted, and how the pilots will elect to route themselves between and through threats" --> Can you name specific examples of such company guidance ?

@summerrain, it will vary substantially upon the operations and the equipment, but a good example is to not penetrate a squall line without NEXRAD (or FSS / ATC WX radar reads). This is because a squall line may have dense returns which might obscure (attenuate) heavy precipitation on the backside.


text:
When must airliners follow airways?
Please give a basic explanation of how commercial aviation traffic is routed for readers without any foreknowledge:

Can commercial aircraft only follow airways like cars on highways ?
Or are airlines free to choose a straight line between airports (excluding probhibited and restricted airspace) ?
Or is it a mix of both ? If so:

When must airliners follow airways ?
And when are they permitted to leave them (absent emergency) ?

What differences (if any) are there between traffic over sea and over land ?

My experience is a bit dated having retired in 1999, but even back then, once we got going, it was common practice to "request direct xxxxxx" where xxxxx was a fix you knew from previous experience was likely to be approved. Approval was usually granted subject to traffic. If it was the middle of the night (flying freight) and the en route controller wasn't working a lot of traffic and sectors had been combined, you could usually expect the request to be approved.

Commercial airliners generally fly IFR as such they do what ATC tells them to do. They are not free to chose their own path. The airway system in the USA and elsewhere on the globe stems from the pre-GPS era when VOR's and other ground based nav aids were the predominant method of navigation. Airways tend to be either between VOR's or between points you can triangulate with a VOR. The system was built around this idea of traffic routing and in the GPS days it has simply stuck. As the FAA shuts down VOR's slowly some of the airways remain and seem a bit arbitrary but GPS allows us to follow them easily.
The FAA has a brief on it here.
Broadly speaking aircraft can request a deviation for weather or other inflight issues like turbulence but aside from that courses are followed as they are assigned.
The FAA has discussed routing directly but until the nextGen system is in place its likely not going to be a widespread reality.

What differences (if any) are there between traffic over sea and over land?

At least over the Atlantic there is the North Atlantic track which is a set of predefined routes. Since there is no ATC over the ocean aircraft do a certain amount of self regulating.

IFR aircraft (which will include nearly all commercial flights) must follow whatever clearance they are given.  The clearance will generally match the flight plan they filed except for some minor vectoring at each end, though it may be amended in the air by ATC, e.g. to deal with weather or conflicting traffic.
So, it really comes down to the flight plan they asked for, which may not be "direct" along the great circle route due to trying to catch better winds, avoiding known weather, avoiding flying through certain airspace or staying within a certain distance of land.  If there are convenient airways that approximate the route desired (and there often are), those can be used and are more likely to be approved, but they can also request their own routes and see what they get.


text:
Is the route which takes the least time always the one with least fuel-burn?
Geographically longer paths may require less fuel than the shortest (great circle) path. But what about time ?
Is the most fuel efficient route always the one that has the aircraft in the air for the shortest amount of time (assuming equal speed setting, e.g. LRC) , or not ?
Users here seem split on this question:

« The most fuel efficient route is the one that has the aircraft in
  the air for the shortest amount of time. » (source)

and:

« It's possible that the route that results in the smallest fuel burn
  is not the necessarily the route that takes the least time. That's true
  more often than not, but if to get the most favorable winds you had to
  stay low, you might burn more fuel. » (source)

So which one is it ?
And, if possible, please provide a clear explanation why.

FYI, I removed the PBN tag from a couple of your questions because it means something very specific about navigation performance, not aircraft performance.

At the extreme, the route with lesser fuel use is the one where you can glide on the maximum distance. Hence not necessarily the quickest. Note that airliners actually glide during their descent, specially on continuous descent approaches.

Generally, the least time in the air the better, but it's mostly a maximum miles per gallon over the earth thing and this depends on winds.  In the time/fuel burn equation, there is always a "sweet spot", a speed/power configuration that gives the most miles per gallon for "air distance" traveled, but how this translates to a maximum distance covered for a given amount of fuel depends on the headwind/tailwind component.
If headwinds are strong, the most efficient speed may be faster than what is optimal for winds that are light or there is a tailwind.  It's easy to see if you take it to the extreme; imagine you are in an airplane that cruises most efficiently at 50 mph and can go 80 mph.  Most miles per gallon air distance is at 50, but if you have a 50 mph headwind, you aren't going anywhere are you, and your actual miles per gallon is 0.   Dial that ridiculous example back to something more reasonable and you can see that there are subtle variations with winds that have to be taken into account.
Generally speaking if there is nil wind or a tail wind, you would fly at the most efficient miles per gallon speed but with a really strong tailwind it might be better to slow down even more for best earth miles per gallon.  If there is a headwind, the optimal speed might be little faster than the max air miles per gallon speed to use the least fuel to get there.
There was an incident once where an RJ crew had to do an IFR diversion with the flaps stuck at 45 degrees and the crew had to decide what speed to fly at, limited to flap extension speed and with all that drag, taking into account whatever headwind/tailwind they had.  They landed at their alternate with 500 lbs on board, about 10 minutes worth.  Talk about pressure to get your calculations right...

Just a quick answer as I don’t have much time: second quote is (usually) corect.
Specific fuel consumption will indicate how many miles you are getting for your galon, but even then  it is not so simple.It all depends on what you consider fixed and what parameters of the flight you are allowed to change:
If you take winds into account, then a longer route might end up getting sooner to the destination, or save up some fuel for the same trip time,should you choose to fly slower.
If cruise level is a factor, then flying higher will get you  there slower (same mach at higher level) but using less fuell overall (better specific consumption)
If you can alter both track to pick better winds, and both cruise level to pick better fuel consumptions, then all bets are off: best fuel is best fuel, not necessary the shortest nor fastest.